With zsh, ligature breaking on prompt behaves oddly

[mitchellh]

Discussion: #4490

I'm not actually sure what's going on here. If you watch the video in #4490 you can see it clearly. You can reproduce it by using zsh (with or without shell integration) and typing the following:

a => b

And then using the left arrow to move your cursor over the "=>". It doesn't break the ligature. But sometimes it does.

I'm not sure what's going on here.

Note that on my machine I couldn't reproduce this with fish or bash, oddly. But I'm not confident in that.